Telephone system



Dec. 19, 1939.

R. HORNICKEL TELEPHONE SY STEM Filed April 14. 1938 RICHARD INVENTOR. HORNICKEL ATTORNEY.

Patented Dec. 19, .1939

- UNITED STATES PATENT. OFFICE TELEPHONE SYSTEM mass Hornickel, Finkenkrng, near-Berlin, Germany,- assignor to Siemens is Halske Aktiengesellschaft, Siemensstadtr near Berlin, Germany Application April 14, 1938,. Serial No. 201,927 In GermanyApril 17, 1937,

comma.

- current is applied to the contacts for a definite 'period, 1. e., temporarily.

The cleaning of contacts has the following purposes: Through the action (if dust, moisture, oxidation, and also through polishing action on 16 the loop contacts, high resistances arise at the points of contact. At the contacts there is formed an exceptionally thin insulating layer which, however, suffices to prevent the passage of weak currents, for example, speech currents. If the 20 contacts lie, for example, in a telephone line, the phenomenon referred to as fading occurs,

whereby the sound intensity is considerably diminished i. e., the intelligibility is reduced, and even quite destroyed. In order to\ preven,t this .one clears the contacts by sending a weak current of the particular potential over the contacts which strikes across the insulating layer formed by the above-mentioned influences and thus geestablishes the uninterrupted current path.

It has been shown that in the known arrangements in which the cleaning current only operates temporarily, i. e., for short periods, on the contacts, this does not suflice to give an eifective cleaning. In particular does this occur inthe case of contacts which, as for example the contacts of automatic telephone switches, are permanently exposed to .violent shocks whereby the contact resistance continually varies. In this case not only the "fading phenomenon above describedoccurs, but undesirable contact noises arise through the influence ofthe cleaning current flowing over the contact and the continual variation in the value of the resistance across the contacts. The invention aims at avoiding this dimculty. This is realized inthat an alternating current is applied to'the contacts'tobecleaned automatically in dependence upon the occurrence of a variation, 1. e.,an increase in the resistance across the contacts and is disconnected after the so ent of the normal resistance value.

The invention will now be described by way of anembodimentfor teiephone systems with reference to the drawing. 7, j

As the source of alternating current used for 54 the cleaning, a thermionic generator G is used in the embodiment, this generating an inaudible high frequency alternating current potential. The thermionic tube 0 is back-coupled over a transformer Uea. 'One branch of the transformer circuit is formed by a resistance R while the 6 other branch is formed by the line F with the contacts i to 4 which are to be cleaned, in series. The resistance R is balanced against the normal resistance of the line F and the contacts I- to 4. If the resistance of the line varies for example 10 on account of the resistance across the contacts I to 4 increasing the balance in the branches of the transformer is disturbed and the back cou pling sets in, i. e., the generator G commences oscillating and transmits the alternating current 15 to the line thereby immediately cleaning the contacts. When the normal contact resistance and thus thebalance in the transformer circuit is re-established the oscillations cease.

In the portion of the telephone line'limited by 20 the condensers CI to C4 there normally flows only the alternating speech current over the selector contacts I, land 2, 4. This portion of the line is cut off from the portion of the line fed with direct current from the batteries-Btl and as Bt! by the condensers l to 4 the cleaning circuit flows from the generator G over the contacts 4, 2, the resonating circuit comprising the choke coil Dr! and the condenser C5 and over the contacts L3 back to the generator G. For preventing the so cleaning potential from passing into the portion of the line leading to subscriber T I the resonance circuit DrI, Ci, which is tuned to the frequency of the alternating current transmitted by the generator G is used, while the resonance circuits 5 D14, C8 and mi, C1 prevent the alternating current passing over the portion of the line to subscriber T2. Since the resonance circuits prevent the cleaning potential from passing into the part of the lines leading to' the subscribers it is pos sible to use voice frequency, i. e., audible alternating currents for the cleaning without there being any fear of disturbing the subscribers or the conversation.

Instead of-transmitting the alternating cur- 45 rent generated by the generator (3 direct to the line-one can use the equipment to connect a com mon cleaning potential produced by a separate source of current for the whole exchange to the lineortothecontactstobecle'aned for-example m less cleaning because the freshly forming contact resistances as a result of the shocks to which they are exposed are immediately nullified and the occurrence of contact noises is prevented at the same time.

What-is claimed is:

1. An arrangement for cleaning the contacts inan electric circuit, comprising an alternating current generating circuit connected to the contacts in the first circuit and normally ineffective, together with means in the generating circuit responsive to a variation in resistance of said first circuit due to poor contacts to supply alternating current to said first circuit momentarily to clean the contacts.

2. In a communication circuit, a plurality of contacts normally having low resistance, a test circuit connected to the communication circuit, means in the test circuit responsive to a variation of the resistance of said contacts for applying an alternating current to the contacts in said first circuit to clean the same, said means responsive to a return to normal of said contact resistance to remove said alternating current.

3. In a communication circuit, a series of contacts, an oscillator circuit coupled to said first circuit, said oscillation circuit normally inoperative but responsive to an increase in the contact resistance of said series of'contacts to start oscillating and supply current to said first circuit to clean said contacts.

4. A communication circuit as claimed in claim 3 in which said oscillation circuit is again rendered inoperative responsive to a return to normal of the contact resistance in said first circuit.

5. A communication circuit having a plurality of contacts included therein, a branch for said circuit containing a circuit balanced to the normal contact resistance of said circuit, and means in the branch circuit responsive to an unbalanced condition due to defective contacts for connecting an alternating current from the branch circuit to the communication circuit to break down the resistance of said defective contacts and restore the balance condition of the circuit.

6. A communication circuit as claimed in claim-5 wherein there are blocking resonant circuits for confining said alternating current to a particular part of said communication circuit.

RICHARD HORNICKEL. 

